U.S. patent number 8,320,769 [Application Number 12/492,399] was granted by the patent office on 2012-11-27 for transverse-mode multiplexing for optical communication systems.
This patent grant is currently assigned to Alcatel Lucent. Invention is credited to Rene'-Jean Essiambre, Roland Ryf, Peter J. Winzer.
United States Patent |
8,320,769 |
Essiambre , et al. |
November 27, 2012 |
Transverse-mode multiplexing for optical communication systems
Abstract
An optical communication system having an optical transmitter
and an optical receiver optically coupled via a multi-path fiber.
The optical transmitter launches, into the multi-path fiber, an
optical transverse-mode-multiplexed (TMM) signal having a plurality
of independently modulated components by coupling each
independently modulated component into a respective transverse mode
of the multi-path fiber. The TMM signal undergoes inter-mode mixing
in the multi-path fiber before being received by the optical
receiver. The optical receiver processes the received TMM signal to
reverse the effects of inter-mode mixing and recover the data
carried by each of the independently modulated components.
Inventors: |
Essiambre; Rene'-Jean (Red
Bank, NJ), Ryf; Roland (Aberdeen, NJ), Winzer; Peter
J. (Aberdeen, NJ) |
Assignee: |
Alcatel Lucent (Paris,
FR)
|
Family
ID: |
42635473 |
Appl.
No.: |
12/492,399 |
Filed: |
June 26, 2009 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20100329671 A1 |
Dec 30, 2010 |
|
Current U.S.
Class: |
398/143; 398/158;
398/140 |
Current CPC
Class: |
H04B
10/677 (20130101); G02B 6/2817 (20130101); G02B
6/34 (20130101); H04B 10/2581 (20130101); H04J
14/04 (20130101); G02B 6/2848 (20130101); G02B
6/14 (20130101) |
Current International
Class: |
H04B
10/12 (20060101); H04B 10/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
US. Appl. No. 12/338,492. cited by other .
"Weakly Guiding Fibers," by D. Gloge, Applied Optics, vol. 10, No.
10, Oct. 1971, pp. 2252-2258. cited by other .
"Polarization Engineering for LCD Projection," by M. G. Robinson,
J. Chen, G. D. Sharp, Wiley, Chichester (England), 2005, Chapter
11, pp. 257-275. cited by other .
"Capacity Enhancement in Coherent Optical MIMO (COMIMO) Multimode
Fiber Links," by Rick C. J. Hsu et al., IEEE Communications
Letters, vol. 10, No. 3, Mar. 2006, pp. 195-197. cited by other
.
"Fundamentals and Challenges of Optical Multiple-Input
Multiple-Output Multimode Fiber Links," by Alireza Tarighat et al.,
Topics in Optical Communications, IEEE Communications Magazine, May
2007, pp. 57-63. cited by other .
"Coherent Optical Multiple-Input Multiple-Output communication," by
Rick C. J. Hsu et al., IEICE Electronics Express, vol. 1, No. 13,
2004, pp. 392-397. cited by other .
"Coherent Optical MIMO (COMIMO)," by Akhil R. Shah et al., Journal
of Lightwave Technology, vol. 23, No. 8, Aug. 2005, pp. 2410-2419.
cited by other .
"High Spectral Efficiency Coherent Optical OFDM for 1 Tb/s Ethernet
Transport," by William Shieh, OWW1.pdf, OCIS codes: (060 2330)
Fiber optics communications; (060. 1660) Coherent Communications,
2009, 3 pages. cited by other .
International Search Report and Written Opinion in
PCT/US2010/038701 Mailing Date: Sep. 13, 2010. cited by other .
Guofeng, Wu., "Methods of Increasing the Bandwidth-Distance Product
for Multimode Fibers in LAN." Journal of Optical Communications,
vol. 29, Jan. 1, 2008, pp. 213-216. cited by other .
U.S. Appl. No. 12/827,284, filed Jun. 30, 2010. cited by other
.
U.S. Appl. No. 12/827,641, filed Jun. 30, 2010. cited by other
.
"Fourier optics", Wikipedia,the free encyclopedia,
"http://en.wikipedia.org/wiki/Fourier.sub.--optics", downloaded
Aug. 31, 2011, 20 pages. cited by other .
Jalali, et al., "Coherent Optical MIMO," 2005; Proc. of SPIE; vol.
5814; pp. 121-127. cited by other .
O'Callaghan, et al., "Spatial Light Modulators with Integrated
Phase Masks for Holographic Data Storage," 2006; IEEE; pp. 23-25.
cited by other .
Lin, et al., "Holographic Fabrication of Photonic Crystals Using
Multidimensional Phase Masks," Dec. 2008; Journal of Applied
Physics. cited by other .
Stuart, H., "Dispersive Multiplexing in Multimode Optical Fiber,"
2000; Science Magazine; pp. 281-283. cited by other.
|
Primary Examiner: Vanderpuye; Kenneth N
Assistant Examiner: Sandhu; Amritbir
Attorney, Agent or Firm: Mendelsohn, Drucker &
Associates, P.C. Mendelsohn; Steve Gruzdkov; Yuri
Claims
What is claimed is:
1. An optical communication system, comprising: a multi-path fiber
that supports a plurality of transverse modes; an optical
transmitter coupled to a first end of the multi-path fiber and
configured to launch an optical transverse-mode-multiplexed (TMM)
signal having N independently modulated components such that, at
the first end, each of the N independently modulated components
corresponds to a respective single transverse mode of the
multi-path fiber, where N is an integer greater than one; and an
optical receiver coupled to a second end of the multi-path fiber
and configured to process the TMM signal received through the
multi-path fiber to recover data carried by each of the N
independently modulated components.
2. The invention of claim 1, wherein: the TMM signal undergoes
inter-mode mixing in the multi-path fiber before being received by
the optical receiver; and the optical receiver is configured to
reverse effects of the inter-mode mixing to recover the data.
3. The invention of claim 1, wherein: the multi-path fiber is a
multimode fiber; and the optical transmitter comprises: a first
plurality of fibers; and an optical mode-coupling (OMC) module
disposed between the first plurality of fibers and the multimode
fiber, wherein: the OMC module is configured to filter each of
optical signals received from the first plurality of fibers to
generate a respective one of N filtered signals and to launch into
the multimode fiber the TMM signal that is based on said filtered
optical signals so that, at the first end, each of the N filtered
optical signals is coupled substantially into a respective single
transverse mode of the multi-path fiber to produce a respective
independently modulated component of the TMM signal.
4. The invention of claim 1, wherein the multi-path fiber is a
multi-core fiber.
5. The invention of claim 1, further comprising an optical add/drop
multiplexer coupled to the multi-path fiber between the optical
transmitter and the optical receiver and configured to perform at
least one of the following: (i) drop from the multi-path fiber all
transverse modes corresponding to a selected common optical
frequency to remove corresponding one or more independently
modulated components from the TMM signal; and (ii) populate in the
multi-path fiber one or more transverse modes corresponding to a
selected vacant optical frequency to add to the TMM signal one or
more independently modulated components, wherein the TMM signal is
also a wavelength-division-multiplexed signal.
6. An optical transmitter, comprising: a first plurality of fibers;
and an optical mode-coupling (OMC) module disposed between the
first plurality of fibers and a multimode fiber, wherein: the
multimode fiber supports a plurality of transverse modes; the OMC
module is configured to filter each of optical signals received
from the first plurality of fibers to generate a respective one of
a plurality of filtered optical signals and to launch into the
multimode fiber an optical transverse-mode multiplexed (TMM) signal
that is based on said filtered optical signals so that, at a
proximate end of the multimode fiber, each of the filtered optical
signals is coupled substantially into a respective single
transverse mode of the multi-path fiber to produce in the multimode
fiber a respective optical component of the TMM signal.
7. The invention of claim 6, wherein: the multimode fiber supports
a total of N transverse modes, where N is an integer greater than
one; the first plurality of fibers comprises N fibers; and at the
proximate terminus of the multimode fiber, the OMC module is
configured to populate the N transverse modes using the N optical
signals received from the N fibers.
8. The invention of claim 6, wherein: at the proximate terminus,
the OMC module is configured to couple: an optical signal from a
first fiber of the first plurality substantially into a first
selected transverse mode of the multimode fiber; and an optical
signal from a second fiber of the first plurality substantially
into a second selected transverse mode of the multimode fiber; and
the first mode is different from the second mode.
9. The invention of claim 6, wherein the OMC module comprises: a
plurality of phase masks, wherein each of said phase masks is (i)
disposed between a respective fiber of the first plurality and the
multimode fiber and (ii) configured to phase-filter an optical beam
produced by said respective fiber; and one or more optical elements
configured to (i) spatially superimpose phase-filtered beams
produced by the phase masks and (ii) apply the superimposed
phase-filtered beams to the proximate terminus of the multimode
fiber to launch the TMM signal.
10. The invention of claim 9, wherein: each of the phase-filtered
beams produces a respective phase/field-strength (PFS) pattern at
the proximate terminus; and said respective PFS pattern
substantially matches a PFS pattern of said respective single
transverse mode.
11. The invention of claim 9, wherein the phase masks are
implemented as different sections of a multi-sectional phase
mask.
12. The invention of claim 6, wherein the OMC module comprises: a
spatial light modulator (SLM) disposed between the fibers of the
first plurality and the multimode fiber, wherein the SLM comprises
a plurality of portions corresponding to the first plurality of
fibers, each of said portions configured to phase-filter an optical
beam produced by the corresponding fiber; and one or more optical
elements configured to (i) spatially superimpose phase-filtered
beams produced by said portions and (ii) apply the superimposed
phase-filtered beams to the proximate terminus of the multimode
fiber to launch the TMM signal.
13. The invention of claim 6, wherein the OMC module comprises: a
volume hologram disposed between the fibers of the first plurality
and the multimode fiber and configured to (i) phase-filter a
plurality of optical beams produced by the first plurality of
fibers and (ii) spatially superimpose phase-filtered beams; and one
or more optical elements configured to apply the superimposed
phase-filtered beams to the proximate terminus of the multimode
fiber to launch the TMM signal.
14. The invention of claim 6, further comprising a plurality of
optical modulators, each coupled to a corresponding fiber of the
first plurality to produce therein the respective optical
signal.
15. The invention of claim 14, further comprising a plurality of
polarization combiners, each disposed between (i) a pair of
modulators from said plurality of modulators and (ii) a
corresponding fiber of the first plurality to perform polarization
multiplexing for optical signals produced by said pair of
modulators and to apply a resulting polarization-multiplexed signal
to the corresponding fiber of the first plurality, wherein the TMM
signal is also a polarization-multiplexed signal.
16. The invention of claim 14, further comprising a plurality of
wavelength multiplexers, each disposed between (i) a corresponding
subset of modulators from said plurality of modulators and (ii) a
corresponding fiber of the first plurality to perform
wavelength-division multiplexing for optical signals produced by
said subset of modulators and to apply a resulting
wavelength-division-multiplexed signal to the corresponding fiber
of the first plurality, wherein the TMM signal is also a
wavelength-division-multiplexed signal.
17. The invention of claim 14, further comprising: a second
plurality of fibers, each configured to feed light into a
corresponding optical modulator of the plurality of optical
modulators to enable said optical modulator to produce the
respective optical signal; and one or more lasers optically coupled
to the second plurality of fibers to generate and apply thereto
said light.
18. A method of generating an optical transverse-mode multiplexed
(TMM) signal, comprising: splitting an optical beam into N
sub-beams, where N is an integer greater than one; modulating each
of the N sub-beams with data to produce N independently modulated
optical signals; and at a proximate terminus of a multi-path fiber,
coupling into the multi-path fiber the N independently modulated
optical signals to produce N independently modulated components of
the TMM signal, wherein: the multi-path fiber supports a plurality
of transverse modes; and each of the N independently modulated
optical signals is coupled into the multi-path fiber such that a
resulting independently modulated component of the TMM signal
corresponds to a respective single transverse mode of the
multi-path fiber at the proximate terminus of the multi-path
fiber.
19. An optical transmitter, comprising: a first plurality of
fibers; and an optical mode-coupling (OMC) module disposed between
the first plurality of fibers and a multimode fiber, wherein: the
multimode fiber supports a plurality of transverse modes; the OMC
module is configured to process optical signals received from the
first plurality of fibers to launch into the multimode fiber an
optical transverse-mode multiplexed (TMM) signal that is based on
said received optical signals; for each fiber of the first
plurality, the OMC module is configured to filter the respective
optical signal received from the fiber such that a resulting
optical component of the TMM signal corresponds to a respective
single transverse mode of the multimode fiber at a proximate
terminus of the multimode fiber; and the OMC module comprises: a
plurality of phase masks, wherein each of said phase masks is (i)
disposed between a respective fiber of the first plurality and the
multimode fiber and (ii) configured to phase-filter an optical beam
produced by said respective fiber; and one or more optical elements
configured to (i) spatially superimpose phase-filtered beams
produced by the phase masks and (ii) apply the superimposed
phase-filtered beams to the proximate terminus of the multimode
fiber to launch the TMM signal.
20. The invention of claim 19, wherein: each of the phase-filtered
beams produces a respective phase/field-strength (PFS) pattern at
the proximate terminus; and said respective PFS pattern
substantially matches a PFS pattern of said respective single
transverse mode.
21. The invention of claim 19, wherein the phase masks are
implemented as different sections of a multi-sectional phase
mask.
22. An optical transmitter, comprising: a first plurality of
fibers; an optical mode-coupling (OMC) module disposed between the
first plurality of fibers and a multimode fiber, wherein: the
multimode fiber supports a plurality of transverse modes; the OMC
module is configured to process optical signals received from the
first plurality of fibers to launch into the multimode fiber an
optical transverse-mode multiplexed (TMM) signal that is based on
said received optical signals; and for each fiber of the first
plurality, the OMC module is configured to filter the respective
optical signal received from the fiber so that a resulting filtered
optical signal is coupled into a respective set of one or more
transverse modes of the multimode fiber at a proximate terminus of
the multimode fiber to produce in the multimode fiber a respective
optical component of the TMM signal; a plurality of optical
modulators, each coupled to a corresponding fiber of the first
plurality to produce therein the respective optical signal; and a
plurality of polarization combiners, each disposed between (i) a
pair of modulators from said plurality of modulators and (ii) a
corresponding fiber of the first plurality to perform polarization
multiplexing for optical signals produced by said pair of
modulators and to apply a resulting polarization-multiplexed signal
to the corresponding fiber of the first plurality, wherein the TMM
signal is also a polarization-multiplexed signal.
23. An optical transmitter, comprising: a first plurality of
fibers; an optical mode-coupling (OMC) module disposed between the
first plurality of fibers and a multimode fiber, wherein: the
multimode fiber supports a plurality of transverse modes; the OMC
module is configured to process optical signals received from the
first plurality of fibers to launch into the multimode fiber an
optical transverse-mode multiplexed (TMM) signal that is based on
said received optical signals; and for each fiber of the first
plurality, the OMC module is configured to filter the respective
optical signal received from the fiber so that a resulting filtered
optical signal is coupled into a respective set of one or more
transverse modes of the multimode fiber at a proximate terminus of
the multimode fiber to produce in the multimode fiber a respective
optical component of the TMM signal; a plurality of optical
modulators, each coupled to a corresponding fiber of the first
plurality to produce therein the respective optical signal; and a
plurality of wavelength multiplexers, each disposed between (i) a
corresponding subset of modulators from said plurality of
modulators and (ii) a corresponding fiber of the first plurality to
perform wavelength-division multiplexing for optical signals
produced by said subset of modulators and to apply a resulting
wavelength-division-multiplexed signal to the corresponding fiber
of the first plurality, wherein the TMM signal is also a
wavelength-division-multiplexed signal.
24. An optical transmitter, comprising: a first plurality of
fibers; and an optical mode-coupling (OMC) module disposed between
the first plurality of fibers and a multimode fiber, wherein: the
multimode fiber supports a plurality of transverse modes; the OMC
module is configured to process optical signals received from the
first plurality of fibers to launch into the multimode fiber an
optical transverse-mode multiplexed (TMM) signal that is based on
said received optical signals; and for each fiber of the first
plurality, the OMC module is configured to filter the respective
optical signal received from the fiber such that a resulting
optical component of the TMM signal corresponds to a respective
single transverse mode of the multimode fiber at a proximate
terminus of the multimode fiber; a plurality of optical modulators,
each coupled to a corresponding fiber of the first plurality to
produce therein the respective optical signal; a second plurality
of fibers, each configured to feed light into a corresponding
optical modulator of the plurality of optical modulators to enable
said optical modulator to produce the respective optical signal;
and one or more lasers optically coupled to the second plurality of
fibers to generate and apply thereto said light.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
The subject matter of this application is related to that of U.S.
patent application Ser. No. 12/492,391, by R.-J. Essiambre, R. Ryf,
and P. Winzer, filed on the same date as the present application,
and entitled "Receiver for Optical Transverse-Mode-Multiplexed
Signals," which application is incorporated herein by reference in
its entirety.
BACKGROUND
1. Field of the Invention
The present invention relates to optical communication equipment
and, more specifically but not exclusively, to the equipment that
enables transverse-mode multiplexing (TMM) in optical communication
systems.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better
understanding of the invention(s). Accordingly, the statements of
this section are to be read in this light and are not to be
understood as admissions about what is in the prior art or what is
not in the prior art.
Wireless communication systems with multiple-input multiple-output
(MIMO) capabilities increase the overall transmission capacity by
exploiting (instead of trying to mitigate) the multi-path delay
spread in a wireless channel. More specifically, wireless MIMO
improves efficiency and reliability through the use of multiple
antennas at the transmitter and receiver. The resulting increase in
average throughput is realized at the expense of greater complexity
in signal processing and hardware implementation, but not at the
expense of additional spectral bandwidth or higher signal
power.
In optical communication systems, a MIMO approach to increasing the
transmission capacity is feasible, for example, because modal
dispersion in a multimode fiber is analogous to multi-path delay in
a wireless transmission medium. Consequently, optical MIMO can be
leveraged to exploit the inherently high transmission capacity of
multimode fibers. However, the hardware for implementing optical
MIMO is not yet sufficiently developed.
SUMMARY
Disclosed herein are various embodiments of an optical
communication system having an optical transmitter and an optical
receiver optically coupled via a multi-path fiber. The optical
transmitter launches, into the multi-path fiber, an optical
transverse-mode-multiplexed (TMM) signal having a plurality of
independently modulated components by selectively coupling each
independently modulated component into a respective single
transverse mode of the multi-path fiber. The TMM signal undergoes
inter-mode mixing in the multi-path fiber before being received by
the optical receiver. The optical receiver processes the received
TMM signal to reverse the effects of inter-mode mixing and recover
the data carried by each of the independently modulated
components.
According to one embodiment, provided is an optical communication
system having: (A) a multi-path fiber that supports a plurality of
transverse modes and (B) an optical transmitter coupled to a first
end of the multi-path fiber and configured to launch an optical TMM
signal having N independently modulated components such that, at
the first end, each of the N independently modulated components
corresponds to a respective single transverse mode of the
multi-path fiber, where N is an integer greater than one. The
optical communication system further has an optical receiver
coupled to a second end of the multi-path fiber and configured to
process the TMM signal received through the multi-path fiber to
recover data carried by each of the N independently modulated
components.
According to another embodiment, provided is an optical transmitter
having: (A) a first plurality of fibers; and (B) an optical
mode-coupling (OMC) module disposed between the first plurality of
fibers and a multimode fiber. The multimode fiber supports a
plurality of transverse modes. The OMC module processes optical
signals received from the first plurality of fibers to launch into
the multimode fiber an optical TMM signal that is based on said
received optical signals. For each fiber of the first plurality,
the OMC module filters the respective optical signal received from
the fiber such that a resulting optical component of the TMM signal
corresponds to a respective single transverse mode of the multimode
fiber at a proximate terminus of the multimode fiber.
According to yet another embodiment, provided is a method of
generating an optical TMM signal. The method has the steps of: (A)
splitting an optical beam into N sub-beams, where N is an integer
greater than one; (B) modulating each of the N sub-beams with data
to produce N independently modulated optical signals; and (C) at a
proximate terminus of a multi-path fiber, coupling into the
multi-path fiber the N independently modulated optical signals to
produce N independently modulated components of the TMM signal. The
multi-path fiber supports a plurality of transverse modes. Each of
the N independently modulated optical signals is coupled into the
multi-path fiber such that a resulting independently modulated
component of the TMM signal corresponds to a respective single
transverse mode of the multi-path fiber at the proximate terminus
of the multi-path fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
Other aspects, features, and benefits of various embodiments of the
invention will become more fully apparent, by way of example, from
the following detailed description and the accompanying drawings,
in which:
FIG. 1 shows a block diagram of an optical communication system, in
which various embodiments of the invention can be practiced;
FIGS. 2A-H show cross-sectional views of optical fibers that can be
used in the system of FIG. 1 according to various embodiments of
the invention;
FIGS. 3A-B illustrate an optical transmitter that can be used in a
node of the system shown in FIG. 1 according to one embodiment of
the invention;
FIGS. 4A-B illustrate an optical mode-coupling (OMC) module that
can be used in the transmitter of FIG. 3 according to one
embodiment of the invention;
FIG. 5 shows a block diagram of an OMC module that can be used in
the transmitter of FIG. 3 according to another embodiment of the
invention;
FIG. 6 shows a block diagram of an OMC module that can be used in
the transmitter of FIG. 3 according to yet another embodiment of
the invention;
FIG. 7 shows a block diagram of an optical transmitter that can be
used in a node of the system shown in FIG. 1 according to another
embodiment of the invention;
FIG. 8 shows a block diagram of an optical transmitter that can be
used in a node of the system shown in FIG. 1 according to yet
another embodiment of the invention;
FIG. 9 shows a block diagram of an optical receiver that can be
used in a node of the system shown in FIG. 1 according to one
embodiment of the invention;
FIGS. 10A-B show a tap module and a coherent detector that can be
used in the optical receiver shown in FIG. 9 according to one
embodiment of the invention;
FIG. 11 show a tap module that can be used in the optical receiver
shown in FIG. 9 according to another embodiment of the invention;
and
FIG. 12 shows a block diagram of an optical receiver that can be
used in a node of the system shown in FIG. 1 according to another
embodiment of the invention.
DETAILED DESCRIPTION
Optical Communication System
FIG. 1 shows a block diagram of an optical communication system
100, in which various embodiments of the invention can be
practiced. System 100 has a plurality of communication nodes 110
interconnected via a network of optical communication links 120.
System 100 further has an optical add/drop multiplexer (ADM) 130,
an optical amplifier 140, and an optical cross-connect 150, all
variously interposed between nodes 110.
Each node 110 has optical multiple-input multiple-output (MIMO)
capabilities achieved through the use of transverse-mode
multiplexing (TMM). Node 110 generally includes an optical
transmitter and an optical receiver (neither is explicitly shown in
FIG. 1) to enable two-way communications between various nodes of
system 100. In addition to TMM multiplexing, an individual node 110
might also use wavelength-division multiplexing (WDM) and/or
polarization multiplexing (PM), or both. Representative embodiments
of an optical transmitter and receiver that can be used in
individual nodes 110 are described in more detail below in the
corresponding subsections of this specification.
Each optical communication link 120 is implemented using one or
more of the following: (i) a single-mode fiber; (ii) a multimode
fiber; (iii) a multi-core fiber; and (iv) a bundle of single-mode
fibers. In one embodiment, a multimode fiber used in link 120
supports between two and about one hundred transverse modes. In an
alternative embodiment, the multimode fiber supports more than one
hundred transverse modes. In one embodiment, each core of a
multi-core fiber used in link 120 supports a single transverse
mode. In an alternative embodiment, some or all cores of the
multi-core fiber support multiple transverse modes, as well as
super-modes of the multi-core fiber taken as a whole.
As used herein, the term "transverse mode" refers to a guided
electromagnetic wave having an electric- or magnetic-field
distribution (hereafter referred to as optical-field distribution),
in a plane perpendicular (i.e. transverse) to the propagation
direction, that is substantially independent of the propagation
distance. More specifically, if a loss or a gain of optical power
in the fiber is factored out, then the mode's optical-field
distributions measured at two different locations along the fiber
will only differ by a factor that reflects the overall phase change
accrued by the mode between those two locations. Each transverse
mode is an eigenmode of the fiber, and different transverse modes
are mutually orthogonal. In general, an optical fiber can support a
fixed number of transverse modes whose optical-field distributions
and propagation constants are unequivocally determined by the
waveguide structure, material properties, and optical frequency
(wavelength). Note that the concept of transverse modes is
applicable to various types of fiber, including the multi-core
fiber. For example, a transverse mode of an individual core in a
multi-core fiber is also a transverse mode of that multi-core fiber
taken as a whole.
In one embodiment, optical add/drop multiplexer 130 is a
reconfigurable add/drop multiplexer. Since link 120 is typically
characterized by a relatively high degree of inter-mode mixing,
node 110 generally needs to receive all transverse modes having the
same optical frequency (wavelength) to properly process a TMM
signal and recover the data carried by that signal. Consequently,
multiplexer 130 is designed to (i) drop from incoming link 120 all
transverse modes having the same optical frequency and/or (ii) add
to outgoing link 120 all populated transverse modes having the same
optical frequency. In other words, multiplexer 130 implements a
conventional WDM add/drop functionality, but acts on a TMM
multiplex of each particular wavelength as a whole.
To support the intended functions, multiplexer 130 employs
narrow-band, interleaver-type optical filters that have
substantially identical transmission characteristics for all
transverse modes. In addition, multiplexer 130 has a relatively low
level of WDM crosstalk (i.e., crosstalk between different optical
frequencies of the WDM multiplex). The latter characteristic can be
achieved, e.g., by: (i) performing the requisite optical filtering
in the single-mode domain prior to a mode-coupling module (see,
e.g., FIG. 8); (ii) using a sinc-type waveform shaping; and/or
(iii) using orthogonal frequency-division multiplexing (OFDM).
If system 100 employs multi-core fibers in links 120, then
multiplexer 130 can be designed to treat a selected set of cores
(which can be all cores of the multi-core fiber or any subset
thereof) as a single entity, thereby adding a wavelength channel to
and/or dropping the wavelength channel from the whole set at the
same time. If link 120 has a relatively low level of crosstalk
between different cores, then multiplexer 130 can be designed to
add communication signals to and/or drop communication signals from
respective individual cores while treating inter-core crosstalk as
noise/impairment.
In various embodiments, optical amplifier 140 can be a lumped
amplifier or a distributed amplifier. In general, system 100 can be
designed to preserve the unitary nature of the mode-mixing matrix
throughout the entire link between two communicating nodes 110.
Consequently, optical amplifier 140 is designed to exhibit
substantially the same gain for all transverse modes of link
120.
In one embodiment, optical amplifier 140 is a relatively long fiber
amplifier (e.g., longer than about 100 m) having (i) a plurality of
active sections and (ii) a plurality of mode scramblers interposed
between the active sections. Each active section provides a
moderate gain (e.g., between about 1 dB and about 5 dB). A mode
scrambler (a.k.a. mode mixer) is an optical device that induces
relatively large mode coupling between different transverse modes.
Ideally, a mode scrambler generates a statistically uniform mode
mix at the output, which mode mix is substantially independent of
the modal distribution received by the mode scrambler at the input.
One skilled in the art will appreciate that the combination of
moderate gain in each active section and relatively frequent mode
scrambling ensures that all transverse modes applied to amplifier
140 are subjected to substantially the same amount of
amplification.
In one embodiment, optical cross-connect 150 is reconfigurable to
enable desired routing of optical signals between different nodes
110. As already mentioned above, to properly decode an individual,
independently modulated component of a TMM multiplex, node 110
generally needs to receive the whole TMM multiplex. Accordingly,
cross-connect 150 is designed to act on the TMM multiplex
corresponding to each wavelength as a whole while performing its
WDM-routing function. One skilled in the art will appreciate that
cross-connect 150 can generally be implemented with many of the
same components as multiplexer 130.
Illustratively, system 100 is shown in FIG. 1 as having four nodes
110, one optical add/drop multiplexer 130, one optical amplifier
140, and one optical cross-connect 150. One skilled in the art will
understand that, in other embodiments, system 100 might have
different numbers of nodes 110, optical add/drop multiplexers 130,
optical amplifiers 140, and/or optical cross-connects 150. One
skilled in the art will further understand that these elements can
generally be arranged and interconnected in a manner different from
that shown in FIG. 1.
Optical Fiber
FIGS. 2A-H show (not to scale) cross-sectional views of optical
fibers that can be used in system 100 according to various
embodiments of the invention. More specifically, the various fibers
shown in FIGS. 2A-H can be used in nodes 110, optical communication
links 120, optical add/drop multiplexers 130, optical amplifiers
140, and/or optical cross-connects 150.
FIG. 2A shows a cross-sectional view of a single-mode fiber 210.
Fiber 210 has a cladding 212 and a core 216. Core 216 has a
relatively small diameter, which causes fiber 210 to support a
single transverse mode for each wavelength from the range of
wavelengths employed in system 100.
FIG. 2B shows a cross-sectional view of a multimode fiber 220.
Fiber 220 has a cladding 222 and a core 226. Fiber 220 differs from
fiber 210 in that core 226 has a larger diameter than core 216. In
various embodiments, the diameter of core 226 is chosen to enable
fiber 220 to support a desired number of transverse modes selected
from a range between two and about one hundred.
FIG. 2C shows a cross-sectional view of a multimode fiber 230.
Fiber 230 has a cladding 232 and a core 236. Core 236 has an even
larger diameter than core 226, which enables fiber 230 to support
more than about one hundred transverse modes.
FIG. 2D shows a cross-sectional view of a multi-core fiber 240.
Fiber 240 has a first (outer) cladding 242 and a second (inner)
cladding 244. Fiber 240 further has a plurality of cores 246
enclosed within inner cladding 244. The diameter of each core 246
can be chosen to cause the core to support either a single
transverse mode or multiple transverse modes.
In one embodiment, fiber 240 is designed for use in optical
amplifier 140. More specifically, inner cladding 244 and/or cores
246 are doped (e.g., with erbium ions) to provide an optically
active medium. Optical pumps of amplifier 140 (not explicitly shown
in FIG. 1) inject optical pump waves into inner cladding 244,
which, due to its index-of-refraction contrast with outer cladding
242, is able to guide those optical pump waves along the
longitudinal axis of fiber 240. The guided optical pump waves
couple from inner cladding 244 into individual cores 246, thereby
providing a source of energy for the amplification of optical
signals guided by the cores. Inner cladding 244 has a diameter that
causes this cladding to function as a multimode core for the
optical pump waves, which ensures that the pump energy is
distributed substantially uniformly among cores 246.
FIG. 2E shows a cross-sectional view of a multi-core fiber 250.
Fiber 250 has a cladding 252 and a plurality of cores 256. Cores
256 are distributed within cladding 252 so that there is a
relatively large separation between the cores. Due to the
relatively large separation, the amount of inter-core crosstalk in
fiber 250 is relatively small, which enables individual cores 256
to function as separate and independent conduits for optical
communication signals. In various embodiments, each individual core
256 can be designed to support either a single transverse mode or
multiple transverse modes.
FIG. 2F shows a cross-sectional view of a multi-core fiber 260.
Fiber 260 has a cladding 262 and a plurality of cores 266. Cores
266 are distributed within cladding 262 so that the separation
between the cores is: (i) sufficiently small to produce a moderate
amount of linear coupling between the cores and (ii) yet
sufficiently large to produce a relatively small amount of
nonlinear coupling between the cores. If each of cores 266 supports
a respective single transverse mode, then these properties of fiber
260 can be used to create a relatively small number of well-defined
and spatially separated transverse modes for the fiber as a
whole.
FIG. 2G shows a cross-sectional view of a multi-core fiber 270.
Fiber 270 has a cladding 272, a first plurality of cores 276, and a
second plurality of cores 278. Cores 276 have a smaller diameter
than cores 278. The separation between the various cores in fiber
270 is similar to the separation used in fiber 260 (FIG. 2F).
One reason for having two different core types in fiber 270 is to
create two types of transverse modes characterized by different
propagation constants. A mismatch in the propagation constants
results in a group-velocity difference, which is generally
beneficial for reducing the detrimental effects of fiber
nonlinearity. For example, a detrimental impact of cross-phase
modulation can be significantly reduced for signals from different
WDM channels when those channels have a relatively large
group-velocity mismatch. In one embodiment, cores 276 and 278 are
distributed throughout cladding 272 to achieve maximum spatial
separation between the cores of the same type.
FIG. 2H shows a cross-sectional view of a multi-core fiber 280.
Fiber 280 has a cladding 282, a first plurality of cores 286, and a
second plurality of cores 288. Although cores 286 and 288 have the
same diameter, they are made of materials having different indices
of refraction. The index-of-refraction difference causes cores 286
and 288 to have different propagation constants, which enables
fiber 280 to reduce the detrimental effects of fiber nonlinearity
via a mechanism that is qualitatively similar to the mechanism
effective in fiber 270 (FIG. 2G).
One skilled in the art will understand that, in addition to the
fibers shown in FIGS. 2A-H, other types of fiber are also possible.
For example, a multi-core fiber having cores of two or more
different sizes that are made of two or more different materials
can be fabricated to implement the features shown in both FIGS. 2G
and 2H.
In one embodiment of system 100, link 120 is implemented, e.g.,
using one of the fibers shown in FIG. 2, so that all relevant
transverse modes have approximately the same propagation speed and
very similar chromatic-dispersion (CD) characteristics. More
specifically, dispersion properties of different transverse modes
can be analyzed using a b-v diagram, in which b is the normalized
propagation constant and v is the normalized optical frequency. A
representative b-v diagram and explanation of parameters b and v
can be found, e.g., in the article by D. Gloge entitled "Weakly
Guiding Fibers," published in Applied Optics, 1971, vol. 10, No.
10, pp. 2252-2258, which article is incorporated herein by
reference in its entirety. Briefly, for a given operating frequency
v.sub.0, the slope of a modal dispersion curve corresponds to the
group velocity of the mode, and the curvature of the dispersion
curve corresponds to the chromatic dispersion of the mode.
To properly invert the mode-mixing matrix corresponding to link
120, a receiver employed in node 110 might need filters/buffers
having a relatively large capacity, e.g., sufficient to cover the
temporal depth equal to the sum of (i) the maximum spread induced
by the effects of chromatic dispersion in link 120 and (ii) the
maximum differential modal delay (DMD) accrued in the link by
different modes. Ideally, one would want some amount of CD for each
of the transverse modes for the same nonlinearity reasons one wants
some amount of CD in a single-mode fiber. Then, configuring link
120 so that all relevant transverse modes have approximately the
same propagation speed and similar CD characteristics helps to
avoid excessive digital processing depth. As a counterexample, let
us assume a 10-Gbaud system (e.g., 100G per mode). For an
intra-modal CD of 20 ps/(km nm) and a link having a length of about
2,000 km, the receiver might need approximately 60 adaptive
T-spaced filter taps. If the differential delay is about 10%, then
the DMD can be as large as approximately 1 ms, while a
significantly smaller processing depth is preferable.
As used herein, the term "multi-path fiber" encompasses both
multimode fibers (e.g., fibers 220 and 230, FIGS. 2B-C) and
multi-core fibers (e.g., fibers 240-280, FIGS. 2D-H).
Optical Transmitter
FIGS. 3A-B illustrate an optical transmitter 300 that can be used
in node 110 (FIG. 1) according to one embodiment of the invention.
More specifically, FIG. 3A shows a block diagram of transmitter
300. FIG. 3B graphically illustrates the operation of an optical
mode-coupling (OMC) module 340 used in transmitter 300.
FIG. 3A illustratively shows transmitter 300 as being coupled to
communication link 120 via an output fiber 350. Fiber 350 is
generally of the same type as the fiber used in the immediately
adjacent section of communication link 120. As already explained
above, communication link 120 can be implemented using any suitable
types of fiber, such as those shown in FIG. 2. The description that
follows is exemplary and corresponds to an embodiment in which
output fiber 350 is similar to fiber 220 (see FIG. 2B). Based on
this description, one skilled in the art will understand how to
design other embodiments of transmitter 300 suitable for coupling
optical communication signals into other types of fiber.
Transmitter 300 has a laser 310 configured to generate an output
light beam of a designated wavelength. A beam splitter 320 spits
the beam generated by laser 310 N ways and couples the resulting N
beams into N single-mode fibers 322, where N is an integer greater
than one. Each fiber 322 directs its respective beam to a
corresponding optical modulator 330, where that beam is modulated
with data supplied to the modulator via a control signal 328. Note
that different modulators 330 or suitable modulator arrangements
can modulate their respective optical beams using different
independent or correlated data streams derived from control signal
328. In a representative configuration, each modulator 330
modulates its optical beam based on a corresponding independent
data stream intended for transmission from transmitter 300 to a
remote receiver. The modulated optical signals produced by
modulators 330 ultimately serve as independently modulated
components of a TMM signal that is applied by OMC module 340 to
fiber 350.
In an alternative embodiment, laser 310 can be directly coupled to
modulators 330 or coupled to the modulators not through fibers, but
through free space.
One function of OMC module 340 is to properly couple the N
modulated optical signals received via N single-mode fibers 332
into fiber 350. More specifically, each modulated optical signal
received by OMC module 340 is coupled substantially into a single
selected transverse mode of fiber 350, with different modulated
optical signals being coupled into different transverse modes. As
used herein, the phrase "to couple a signal substantially into a
single transverse mode" can have two possible, but not mutually
exclusive, meanings. According to the first meaning, this phrase
means that at least about 50% (and perhaps more than about 80% or
90%) of the total energy of the signal couples into one selected
transverse mode. According to the second meaning, this phrase means
that, for the portion of the total energy of the signal that is
coupled into the multimode fiber, at least about 50% (and perhaps
more than about 80% or 90%) of that portion goes into one selected
transverse mode, while the remainder of that portion goes into
other transverse modes. When an individual optical signal is
coupled (e.g., by OMC module 340) substantially into a single
transverse mode of the multimode fiber (e.g., fiber 350) at a
proximate terminus of that fiber and is thereby transformed into an
optical component of a TMM signal that is launched into the
multimode fiber, it is said that that optical component of the TMM
signal "corresponds to a single transverse mode of the multimode
fiber at the proximate terminus of the multimode fiber."
In one embodiment of transmitter 300, the number N is chosen to be
the same as the total number of transverse modes supported by fiber
350. In other words, this embodiment of transmitter 300 employs OMC
module 340 that is capable of populating each and every of the
transverse modes of multimode fiber 350 with a respective
independently modulated optical signal.
FIG. 3B graphically depicts the optical beam shaping performed by
OMC module 340. More specifically, different panels of FIG. 3B show
various phase/field-strength (PFS) patterns that can be produced by
OMC module 340 at a terminus 348 of fiber 350, with different
panels corresponding to different optical channels of the OMC
module. Each PFS pattern is represented in FIG. 3B using a color
scheme in which: (i) the degree of color saturation represents the
optical-field strength and (ii) the color itself represents the
phase of the optical field. For example, the light red color
corresponds to lower optical-field strength than the dark red
color. A rainbow-like change in color from blue to red represents a
continuous change in the phase from -.pi. to +.pi..
For each optical channel of OMC module 340, the PFS pattern
produced by that channel at terminus 348 of fiber 350 substantially
matches the PFS pattern of the transverse mode assigned to that
channel. One skilled in the art will understand that the
above-indicated mode-coupling loss might be caused by a mismatch
between these PFS patterns. The light energy corresponding to the
mode-coupling loss might be parasitically coupled into other
transverse modes of fiber 350 and/or be altogether rejected by the
fiber.
As used herein, the term "substantially matches" means that the
difference between the PFS pattern generated by the optical channel
and the PFS pattern of the corresponding transverse mode is
relatively small and satisfies at least one of two possible, but
not mutually exclusive, criteria. According to the first criterion,
the difference is so small that at least about 50% (and perhaps
more than about 80% or 90%) of the total energy of the PFS pattern
generated by the optical channel couples into the corresponding
transverse mode. According to the second criterion, the difference
is so small that, for the portion of the total energy of the PFS
pattern generated by the optical channel that is coupled into the
multimode fiber, at least about 50% (and perhaps more than about
80% or 90%) of that portion goes into the corresponding transverse
mode, while the remainder of that portion goes into other
transverse modes.
Different transverse modes corresponding to different PFS patterns
of FIG. 3B are labeled using the following notation. The letters
"LP" stand for "linearly polarized." The numbers that follow the
"LP" in the label give, in the designated order, the values of two
quantized parameters. For each transverse mode, the first quantized
parameter gives the number of 2.pi.-sized phase increments per one
azimuthal rotation about the fiber axis, and the second quantized
parameter gives the number of .pi.-sized phase increments over the
fiber radius. For example, the transverse mode designated as LP01
has (i) no azimuthal phase increments and (ii) one radial phase
increment. Similarly, the transverse mode designated as LP32 has
(i) three azimuthal phase increments and (ii) two radial phase
increments.
If OMC module 340 has eight optical channels, then the following
representative mode assignment can be used: (Ch.1)-LP01;
(Ch.2)-LP11; (Ch.3)-LP21; (Ch.4)-LP02; (Ch.5)-LP31; (Ch.6)-LP12;
(Ch.7)-LP41; and (Ch.8)-LP22. On skilled in the art will understand
that other mode assignments, according to which each optical
channel of OMC module 340 is configured to produce a PFS pattern
that substantially matches the PFS pattern of the assigned
transverse mode of fiber 350, can similarly be used without
departing from the scope and principle of the invention.
One skilled in the art will understand that FIG. 3B corresponds to
one possible transverse-mode basis set, and that other basis sets,
each comprising a plurality of mutually orthogonal transverse modes
can similarly be used to implement OMC module 340.
FIGS. 4A-B illustrate an OMC module 400 that can be used as OMC
module 340 (FIG. 3) according to one embodiment of the invention.
More specifically, FIG. 4A shows a block diagram of OMC module 400.
FIG. 4B shows phase masks 420 that can be used in OMC module
400.
OMC module 400 has two optical channels and, as such, is shown as
being coupled to two input fibers 332 (see also FIG. 3A). One
skilled in the art will understand that OMC module 400 can be
modified in a straightforward manner to have three or more optical
channels. More specifically, a new optical channel can be created
by adding a set of optical elements similar to that used to form
Channel 2 in OMC module 400.
OMC module 400 has two lenses 410, each of which collimates a
respective diverging light beam applied to the OMC module by a
respective one of fibers 332. Each of the resulting collimated
beams passes through a respective one of phase masks 420 to create
a corresponding phase-filtered beam 422. A plurality of mirrors 430
then spatially superimpose the two phase-filtered beams 422 and
direct a resulting "superimposed" beam 432 toward fiber 350. Note
that mirror 430.sub.4 is a partially transparent mirror, while
mirrors 430.sub.1-430.sub.3 are regular non-transparent mirrors.
Two lenses 442 and 446 and an aperture 444 are used to compress
(i.e., reduce the size of) and spatially filter beam 432 to produce
an output beam 452 that impinges on terminus 348 of fiber 350 and
creates an intended superposition of the PFS patterns shown in FIG.
3B.
Depending on the transverse modes assigned to the two optical
channels of OMC module 400, phase masks 420.sub.1 and 420.sub.2 are
appropriately chosen, e.g., from the assortment of phase masks
shown in FIG. 4B. For example, if a particular optical channel of
OMC module 400 is assigned the LP11 mode of fiber 350, then the
phase mask labeled LP11 in FIG. 4B is used as phase mask 420 in
that optical channel. Similarly, if a particular optical channel of
OMC module 400 is assigned the LP21 mode of fiber 350, then the
phase mask labeled LP21 in FIG. 4B is used as phase mask 420 in
that optical channel, and so on. The combined effect of phase
filtering imposed by phase mask 420 and spatial filtering imposed
by aperture 444 is that the optical channel creates at terminus 348
of fiber 350 an intended one of the PFS patterns shown in FIG. 3B,
thereby efficiently coupling the optical signal from the optical
channel into the corresponding transverse mode of the fiber.
Note that some of the phase masks shown in FIG. 4B are binary phase
masks (i.e., phase masks that can locally impose only one of two
possible phase shifts, e.g., either 0 or .pi.). In particular,
phase masks corresponding to the LP01, LP02, and LP03 modes are
binary phase masks. The remaining phase masks shown in FIG. 4B are
"analog" phase masks because different portions of the phase mask
can impose phase shifts selected from a continuous phase-shift
range. Analog phase masks are shown in FIG. 4B using a color
scheme, in which: (i) different colors represent different phase
shifts in a continuous 2.pi. interval and (ii) different bands of
the same color might represent phase shifts that differ from each
other by an integer multiple of 2.pi..
In one embodiment, OMC module 400 might employ a single, relatively
large, continuous phase mask in place of two separate phase masks
420.sub.1 and 420.sub.2. This relatively large, continuous phase
mask, hereafter termed "a multi-sectional phase mask," might
contain, in its different sections (portions), two or more phase
masks from FIG. 4B. These sections of the multi-sectional phase
mask are arranged so that one section serves as phase mask
420.sub.1 and another section serves as phase mask 420.sub.2.
FIG. 5 shows a block diagram of an OMC module 500 that can be used
as OMC module 340 (FIG. 3) according to another embodiment of the
invention. OMC module 500 is generally analogous to OMC module 400
(FIG. 4) and uses many of the same elements, such as lenses 410,
442, and 446, mirrors 430, and aperture 444. The description of
these elements is not repeated here. Instead, the description of
OMC module 500 that follows focuses on differences between OMC
modules 400 and 500.
One difference between OMC modules 400 and 500 is that the latter
employs a spatial light modulator (SLM) 520 instead of phase masks
420. In one embodiment, SLM 520 is a liquid-crystal-on-silicon
(LCOS) SLM. A representative LCOS SLM that can be used as SLM 520
is described, e.g., in "Polarization Engineering for LCD
Projection," by M. G. Robinson, J. Chen, G. D. Sharp, Wiley,
Chichester (England), 2005, Chapter 11, pages 257-275, the
teachings of which are incorporated herein by reference in their
entirety. LCOS SLMs that can be adapted for use as SLM 520 are also
disclosed, e.g., in U.S. Pat. Nos. 7,268,852, 6,940,577, and
6,797,983, all of which are incorporated herein by reference in
their entirety. A suitable LCOS SLM that can be used as SLM 520 is
manufactured by JVC Corporation and is commercially available as
part of JVC Projector Model DLA-HD2K.
SLM 520 has two areas 524.sub.1 and 524.sub.2 configured to perform
the requisite phase filtering for Channel 1 and Channel 2,
respectively, of OMC module 500. More specifically, area 524.sub.1
is configured to display a spatial-modulation pattern that produces
phase filtering similar to that of phase mask 420.sub.1 (see FIG.
4A). Similarly, area 524.sub.2 is configured to display a
spatial-modulation pattern that produces phase filtering similar to
that of phase mask 420.sub.2 (also see FIG. 4A).
Since SLM 520 is a reconfigurable device, it can be used to
dynamically change or adjust the patterns displayed in its various
areas, e.g., areas 524.sub.1 and 524.sub.2. This feature can be
useful, e.g., to enable a relatively easy change in the
transverse-mode assignment for different optical channels of OMC
module 500 and/or to maintain optimal optical coupling for
different optical channels of the OMC module under changing
operating conditions that might cause corresponding changes in the
relevant characteristics of multimode fiber 350.
OMC module 500 uses two polarization beam splitters 528 to
appropriately direct the collimated beams produced by lenses 410
toward SLM 520 and the phase-filtered beams produced by the SLM
toward mirrors 430. In one embodiment, a quarter-wave plate (not
explicitly shown in FIG. 5) can be inserted between polarization
beam splitter 528 and SLM 500 to appropriately rotate the
polarization of the beam transmitted through that plate to enable
the polarization beam splitter to direct the collimated beam toward
the SLM while directing the phase-filtered beam toward mirrors 430.
In an alternative embodiment, SLM 500 can be designed to rotate the
polarization of the reflected light to enable polarization beam
splitters 528 to do the same beam routing.
In one embodiment, OMC module 500 can be used to produce a TMM
signal that is also polarization multiplexed. In particular, if SLM
520 itself is substantially polarization insensitive, then the same
SLM can be used to process both polarizations used for polarization
multiplexing.
FIG. 6 shows a block diagram of an OMC module 600 that can be used
as OMC module 340 (FIG. 3) according to yet another embodiment of
the invention. OMC module 600 is generally functionally analogous
to OMC modules 400 and 500 (FIGS. 4 and 5). However, OMC module 600
differs from OMC modules 400 and 500 in that it employs a volume
hologram 620, which can generally be viewed as a three-dimensional
phase mask. Note that, in contrast to volume hologram 620, phase
mask 420 and SLM 520 can generally be viewed as thin-film or
two-dimensional phase masks.
In OMC module 600, volume hologram 620 performs at least two
different functions. The first of these functions is a
phase-filtering function similar to that of phase masks 420 in OMC
module 400 and of SLM 520 in OMC module 500. The second of these
functions is a beam-combining function similar to that of mirrors
430. Volume hologram 620 is capable of applying the requisite
different phase filtering to different optical signals received
from fibers 332 because the corresponding optical beams traverse
different sub-volumes of the volume hologram. Also for this reason,
volume hologram 620 is capable of changing the propagation
direction for different optical signals by different amounts.
Volume holograms are known in the art and are described in more
detail, e.g., in U.S. Pat. Nos. 7,416,818, 7,323,275, and
6,909,528, all of which are incorporated herein by reference in
their entirety.
OMC module 600 is illustratively shown as having three optical
channels. One skilled in the art will understand that OMC module
600 can be modified in a straightforward manner to have a different
number of optical channels.
FIG. 7 shows a block diagram of an optical transmitter 700 that can
be used in node 110 (FIG. 1) according to another embodiment of the
invention. Similar to transmitter 300 (FIG. 3), transmitter 700 has
the capability of selectively coupling independently modulated
optical signals into respective transverse modes of an output fiber
(i.e., fiber 750). However, additionally, transmitter 700 has
polarization-multiplexing (PM) capabilities enabled by the use, in
a modulation and polarization-multiplexing (MPM) module 712 of the
transmitter, of a polarization beam splitter 714 and polarization
combiners 734. Transmitter 700 is illustratively shown as having
three optical channels per polarization. One skilled in the art
will understand that transmitter 700 can be modified in a
straightforward manner to have a different number of channels per
polarization (e.g., two or more than three).
Transmitter 700 has a laser 710 configured to generate an output
light beam of a designated wavelength. Laser 710 feeds MPM module
712, in which polarization beam splitter 714 spits the beam
generated by the laser into beams 716.sub.1 and 716.sub.2 having
mutually orthogonal polarizations. MPM module 712 further has two
power splitters 720, each of which splits the respective polarized
beam received from polarization beam splitter 714 three ways and
couples the resulting three beams into three single-mode fibers
722. Each fiber 722 directs its respective beam to the
corresponding optical modulator 730, where that beam is modulated
with data supplied to the modulator via a control signal 728. The
modulated optical signals produced by modulators 730 are coupled
into single-mode fibers 732 and directed to the corresponding
polarization combiner 734. Each polarization combiner 734 combines
the two received orthogonally polarized signals into a
corresponding PM signal and then directs that PM signal, via a
respective single-mode fiber 736, to an OMC module 740.
In an alternative embodiment, transmitter 700 can use direct
optical coupling or through-free-space optical coupling instead of
or in addition to at least some of the fiber coupling shown in FIG.
7.
OMC module 740 of transmitter 700 is generally analogous to OMC
module 340 of transmitter 300 (FIG. 3) and serves to properly
couple the three received PM signals into fiber 750. More
specifically, each PM signal received by OMC module 740 is coupled
into a selected transverse mode of fiber 750, with different PM
signals being coupled into different transverse modes. For an
individual PM signal (which has two orthogonally polarized
components), each of its polarization components is subjected to
substantially the same phase filtering in OMC module 740. In
various embodiments, OMC module 740 can be implemented similar to
OMC modules 400, 500, and 600 (see FIGS. 4-6). However, in
designing OMC module 740, special attention is given to its
polarization-handling characteristics to enable said OMC module to
be substantially polarization insensitive.
FIG. 8 shows a block diagram of an optical transmitter 800 that can
be used in node 110 (FIG. 1) according to yet another embodiment of
the invention. Similar to each of transmitters 300 and 700 (see
FIGS. 3 and 7), transmitter 800 has the capability of selectively
coupling optical communication signals into various transverse
modes of an output fiber (i.e., fiber 850). Similar to transmitter
700, transmitter 800 has polarization-multiplexing capabilities
enabled by the use of three MPM modules 812, each of which is
analogous to MPM module 712 (see FIG. 7). However, additionally,
transmitter 800 has WDM capabilities. Thus, transmitter 800 uses
three different types of multiplexing: transverse-mode multiplexing
(TMM), polarization multiplexing (PM), and wavelength-division
multiplexing (WDM).
Transmitter 800 has three TMM channels, three WDM channels, and two
PM channels, which enables the transmitter to generate a TMM signal
having up to eighteen independently modulated optical communication
signals (up to six per excited transverse mode). One skilled in the
art will understand that transmitter 800 can be modified in a
relatively straightforward manner to be able to generate a
different number of independently modulated components and couple
them into selected transverse modes of fiber 850 in any desired
manner.
Each WDM channel of transmitter 800 has a respective laser 810 that
generates a designated wavelength and applies it to a respective
MPM module 812. The three outputs of MPM module 812 are applied to
an optical filter (OF) 818 that performs relatively tight bandpass
filtering intended to reduce crosstalk between optical signals
corresponding to different WDM channels. The filtered signals are
WDM multiplexed in multiplexers 826 and the resulting WDM signals
are directed, via single-mode or integrated fibers 836, to OMC
module 840.
OMC module 840 of transmitter 800 is generally analogous to OMC
module 340 of transmitter 300 (FIG. 3) and serves to properly
couple the three received WDM signals into fiber 850. More
specifically, each WDM signal received by OMC module 840 is coupled
into a selected transverse mode of fiber 850, with different WDM
signals being coupled into different transverse modes. Although the
PFS pattern corresponding to a transverse mode of a multimode fiber
depends on the wavelength, the typical spectral bands used in WDM
systems are relatively narrow, which in practice enables the use of
the same phase mask for all WDM channels. For example, a spectral
band centered at about 1550 nm and having a total width of about
100 nm has only about 6% variance in the carrier frequency across
the entire spectral band. Due to this relatively small variance,
the phase mask designed for a wavelength located near the middle of
the spectral band will work sufficiently well for all wavelengths
in the band. Consequently, in various embodiments, OMC module 840
can be implemented similar to OMC modules 400, 500, and 600 (see
FIGS. 4-6).
Optical Receiver
It is known in the art that transverse modes of a multi-path fiber
undergo inter-mode mixing as they propagate along the length of the
fiber. In general, the effects of inter-mode mixing are stronger in
a multimode fiber. However, a multi-core fiber having relatively
closely spaced cores might also exhibit relatively strong
inter-mode mixing (e.g., inter-core crosstalk). As a result, even
if the communication signal is coupled into a particular single
transverse mode at the front end of the multi-path fiber, other
transverse modes will have contributions from that communication
signal at the remote end of the fiber. Hence, a significant amount
of signal processing needs to be performed at the receiver to fully
recover the data carried by different independently modulated
components of a TMM signal. In general, to decode N independently
modulated components of a TMM signal, the receiver needs to obtain
at least N independent samples of the signal. The signal processing
applied to these samples is generally based on
matrix-diagonalization algorithms aimed at reversing the effects of
inter-mode mixing in the multi-path fiber.
FIG. 9 shows a block diagram of an optical receiver 900 that can be
used in node 110 (FIG. 1) according to one embodiment of the
invention. Receiver 900 can be configured, e.g., to receive an
input TMM signal 902 from link 120. TMM signal 902 is applied to a
tap module 910 that produces K samples 912 of that TMM signal,
where K is a positive integer greater than one. Each sample 912 is
coherently detected by a corresponding coherent detector 930 using
a local oscillator (LO) signal 922 supplied by an LO source 920.
The detection results generated by coherent detector 930 from
sample 912, e.g., an in-phase component I and a quadrature-phase
component Q of the sample, are applied to the digital signal
processor (DSP) 940. For each signaling interval (e.g., bit
period), DSP 940 appropriately processes a full set of detection
results generated by coherent detectors 930.sub.1-930.sub.K to
generate an output data stream 942. Provided that tap module 910
produces enough samples of TMM signal 902, DSP 940 is able to
recover and output via stream 942 all the data that have been
originally encoded by the remote transmitter onto the TMM signal
that is received by receiver 900 as TMM signal 902.
One skilled in the art will understand that one function of DSP 940
is to invert the mode-mixing matrix corresponding to link 120. In
general, link conditions change over time, thereby causing the
mode-mixing matrix to change as well, usually on a millisecond time
scale or slower. In one embodiment, DSP 940 is configured to
adaptively follow link-condition variations. For example, DSP 940
can employ, as known in the art, blind adaptation algorithms to
learn the link conditions and to adapt to them. Alternatively or in
addition, from time to time, a controller 950 coupled to DSP 940
might request that the remote transmitter send to receiver 900 a
training sequence for the DSP to obtain the current mode-mixing
matrix. A representative training sequence applied by the remote
transmitter to link 120 might have a TMM signal in which different
transverse modes are sequentially excited in a known order so that
only one transverse mode is excited at any given time. The signal
processing implemented in DSP 940 might also compensate for certain
nonlinear impediments, such as the phase shifts induced by
self-modal and cross-modal fiber nonlinearity.
FIGS. 10A-B shows representative modules that can be used in
receiver 900 (FIG. 9) according to one embodiment of the invention.
More specifically, FIG. 10A shows a block diagram of a tap module
1010 that can be used as tap module 910. FIG. 10B shows a block
diagram of a coherent detector 1030 that can be used as coherent
detector 930.
Referring to FIG. 10A, tap module 1010 receives TMM signal 902 via
a multimode fiber 1002. A collimation lens 1004 collimates the
diverging light beam produced by fiber 1002 and directs the
resulting collimated beam toward K-1 partially transparent mirrors
1006.sub.1-1006.sub.K-1 and a terminal non-transparent mirror
1006.sub.K. In one implementation, different mirrors 1006 have a
reflectivity that causes beams 1012.sub.1-1012.sub.K reflected from
the mirrors to have approximately the same intensity.
Referring to FIG. 10B, detector 1030 receives LO signal 922 from LO
source 920 via a single-mode fiber 1016. A collimation lens 1018
collimates the diverging light beam produced by fiber 1016 and
directs the resulting collimated beam toward a phase mask 1020.
Phase mask 1020 is generally analogous to phase mask 420 (see FIGS.
4A-B). More specifically, phase mask 1020 produces a phase-filtered
beam 1022 having a PFS pattern that is a magnified (enlarged)
version of the PFS pattern corresponding to a selected transverse
mode of multimode fiber 1002 (FIG. 10A). As already indicated
above, FIG. 4B shows an assortment of phase masks, each of which is
suitable for use as phase mask 1020. When each detector 930 in
receiver 900 is implemented using detector 1030, different
instances of detector 1030 in the receiver generally have different
phase masks 1020 (e.g., different phase masks selected from the
assortment shown in FIG. 4B). In various embodiments, these
different phase masks 1020 can be implemented as different sections
of a multi-sectional phase mask or using different portions of an
SLM similar to SLM 520 (FIG. 5).
Phase-filtered beam 1022 and beam 1012 (that carries TMM sample
912, see FIGS. 9 and 10A) are applied to a 2.times.4 optical hybrid
1026, where they beat against each other to generate four
interference signals 1032.sub.1-1032.sub.4. Each of interference
signals 1032.sub.1-1032.sub.4 is applied to a corresponding
photo-detector (e.g., photodiode) 1034 that converts it into a
corresponding electrical signal. The electrical signals generated
by photo-detectors 1034 are digitized and directed for further
processing in DSP 940. 2.times.4 optical hybrids that can be used
as hybrid 1026 in detector 1030 are known in the art, with
representative examples being disclosed, e.g., in (i) U.S. Patent
Application Publication No. 2007/0297806 and (ii) U.S. patent
application Ser. No. 12/338,492, filed Dec. 18, 2008, both of which
are incorporated herein by reference in their entirety.
The electric fields E.sub.1-E.sub.4 of interference signals
1032.sub.1-1032.sub.4, respectively, are given by Eq. (1):
.function..times..times..times..times..times..times..times..times..times.-
e.times..times..pi..times.e.times..times..pi. ##EQU00001## where
E.sub.S and E.sub.LO are the electric fields of optical signals
1012 and 1022, respectively. Note that Eq. (1) holds for every
point of a transverse cross-section of beam 1032. This means that
detector 1030 measures not only how beams 1012 and 1022 beat
against each other in time, but also how they beat against each
other in space. Furthermore, due to the mutual orthogonality of
different transverse modes of fiber 1016, different instances of
detector 1030 having different phase masks 1020 effectively measure
the electric fields corresponding to different transverse modes of
TMM signal 902. One skilled in the art will understand that, if
K.gtoreq.N, then coherent detectors 930 (or 1030) generate enough
sampling data to enable DSP 940 to properly invert the mode-mixing
matrix corresponding to communication link 120 and recover the data
carried by the independently modulated components of the TMM signal
transmitted therethrough from a remote transmitter (e.g.,
transmitter 300) to receiver 900.
FIG. 11 shows a block diagram of a tap module 1110 that can be used
as tap module 910 according to another embodiment of the invention.
Tap module 1110 has a multimode fiber 1102 through which it
receives TMM signal 902. Along the length of fiber 1102, tap module
1110 has K multimode-fiber (MMF) couplers 1106.sub.1-1106.sub.K and
K-1 mode scramblers 1108.sub.2-1108.sub.K. Each MMF coupler 1106 is
a fiber tap that branches off a portion of TMM signal 902 and
couples that portion into a corresponding single-mode fiber 1110.
Signals 1112.sub.1-1112.sub.K carried by fibers
1110.sub.1-1110.sub.K serve as samples 912.sub.1-912.sub.K,
respectively, in receiver 900.
Each signal 1112 is indicative of the linear combination of the
transverse modes that is present in multimode fiber 1102 at the
location of the corresponding MMF coupler 1106. Since mode
scramblers 1108.sub.2-1108.sub.K mix up the transverse modes
between MMF couplers 1106.sub.1-1106.sub.K, each of signals
1112.sub.1-1112.sub.K is indicative of a different linear
combination of the transverse modes in multimode fiber 1102. One
skilled in the art will understand that, if K.gtoreq.N, then
coherent detectors 930 generate enough sampling data to enable DSP
940 to properly invert the mode-mixing matrix corresponding to
communication link 120 and recover the data carried by the
independently modulated components of the TMM signal transmitted
therethrough from a remote transmitter (e.g., transmitter 300) to
receiver 900.
In one embodiment, receiver 900 having tap module 1110 can use, as
coherent detectors 930, the coherent detectors designed for the
detection of PM signals. Coherent detectors for the detection of PM
signals are known in the art and disclosed, e.g., in the
above-cited U.S. Patent Application Publication No. 2007/0297806
and U.S. patent application Ser. No. 12/338,492. One skilled in the
art will understand that receiver 900, employing tap module 1110
and a plurality of coherent detectors for the detection of PM
signals, is capable of appropriately detecting optical signals that
are produced with the use of both TMM and PM multiplexing. One
skilled in the art will further understand that a WDM receiver
capable of appropriately detecting optical signals that are
produced with the use of all three of the above-mentioned types of
multiplexing (i.e., TMM, PM, and WDM) can be constructed by
deploying one receiver 900 having both TMM and PM capabilities for
each WDM channel of the WDM receiver.
FIG. 12 shows a block diagram of an optical receiver 1200 that can
be used in node 110 (FIG. 1) according to another embodiment of the
invention. Receiver 1200 receives a TMM signal 1201 (e.g., from
link 120) via a multimode fiber 1202. A collimation lens 1204.sub.1
collimates the diverging light beam produced by fiber 1202 and
directs a resulting collimated beam 1205 toward beam splitters
1206.sub.1-1206.sub.4. In one embodiment, each beam splitter 1206
is a semitransparent mirror.
Receiver 1200 also has an LO source 1220 that passes its output
through a collimation lens 1204.sub.2 to form a collimated LO beam
1221. Similar to beam 1205, LO beam 1221 is also directed toward
beam splitters 1206.sub.1-1206.sub.4. A 90-degree phase shifter
1208 located between beam splitters 1206.sub.1 and 1206.sub.2
introduces a 90-degree phase shift into the beam transmitted
therethrough.
Beam splitters 1206.sub.1-1206.sub.4 appropriately split beams 1205
and 1221 into a plurality of sub-beams and then recombine some of
these sub-beams to generate four mixed optical beams that impinge
onto pixelated receiving surfaces of four arrayed detectors (e.g.,
CCDs) 1230.sub.1-1230.sub.4, where the mixed optical beams produce
the corresponding interference patterns. Each arrayed detector 1230
operates at a sufficiently high speed that enables it to capture
and output data corresponding to at least one interference pattern
per signaling interval (e.g., symbol period) of TMM signal 1201.
Each interference pattern is created at the pixelated receiving
surface of arrayed detector 1230 by beating against each other the
reference field generated by LO source 1220 and the optical field
of TMM signal 1201. Arrayed detector 1230 captures the interference
pattern by measuring the light intensity of the pattern at the
various pixels of the arrayed detector, thereby creating a
two-dimensional cross-sectional intensity profile of the mixed
beam.
The data corresponding to the four interference patterns detected
by arrayed detectors 1230.sub.1-1230.sub.4 are supplied to a DSP
1240 for processing. If arrayed detectors 1230.sub.1-1230.sub.4
have sufficiently high resolution (e.g., a sufficiently large
number of relatively small pixels), then DSP 1240 receives enough
data to determine, from the four interference patterns, the modal
composition of TMM signal 1201. Herein, the term "modal
composition" refers to a representation of TMM signal 1201 in terms
of transverse modes of multimode fiber 1202. Typically, such a
representation is a linear combination of appropriately weighted
transverse modes. The knowledge of the modal composition then
enables the DSP to properly invert the mode-mixing matrix
corresponding to communication link 120 and recover the data
carried by the independently modulated components of the TMM signal
transmitted therethrough from a remote transmitter (e.g.,
transmitter 300) to receiver 1200. DSP 1240 outputs the recovered
data via a data stream 1242.
One skilled in the art will understand that arrayed detectors
1230.sub.2 and 1230.sub.4 are optional and are used in receiver
1200 to implement a balanced detection scheme similar to that
implemented in detector 1030. More specifically, the four
interference patterns detected by arrayed detectors
1230.sub.1-1230.sub.4 are processed by DSP 1240 to generate two
cross-sectional maps of TMM signal 1201. The first cross-sectional
map is an in-phase map of TMM signal 1201, and the second
cross-sectional map is a quadrature-phase map of the TMM signal.
Having the in-phase and quadrature phase maps of TMM signal 1201
might be advantageous because DSP 1240 can use these maps to make
the determination of the modal composition of the TMM signal
faster, more accurate, and/or more efficient.
In various embodiments, receiver 1200 might include additional
optical components to enable the use of fewer than four separate
arrayed detectors. For example, in one embodiment, receiver 1200
might have two relatively large arrayed detectors, wherein: (i) the
first detector is partitioned so that one portion of the first
detector serves as arrayed detector 1230.sub.1 while another
portion of the first detector serves as arrayed detector 1230.sub.2
and (ii) the second detector is similarly partitioned so that one
portion of the second detector serves as arrayed detector
1230.sub.3 while another portion of the second detector serves as
arrayed detector 1230.sub.4. In an alternative embodiment, receiver
1200 might have one very large arrayed detector that is partitioned
into four portions, each serving as a corresponding one of
detectors 1230.sub.1-1230.sub.4.
While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications of the
described embodiments, as well as other embodiments of the
invention, which are apparent to persons skilled in the art to
which the invention pertains are deemed to lie within the principle
and scope of the invention as expressed in the following
claims.
Unless explicitly stated otherwise, each numerical value and range
should be interpreted as being approximate as if the word "about"
or "approximately" preceded the value of the value or range.
It will be further understood that various changes in the details,
materials, and arrangements of the parts which have been described
and illustrated in order to explain the nature of this invention
may be made by those skilled in the art without departing from the
scope of the invention as expressed in the following claims.
Although the elements in the following method claims, if any, are
recited in a particular sequence with corresponding labeling,
unless the claim recitations otherwise imply a particular sequence
for implementing some or all of those elements, those elements are
not necessarily intended to be limited to being implemented in that
particular sequence.
Reference herein to "one embodiment" or "an embodiment" means that
a particular feature, structure, or characteristic described in
connection with the embodiment can be included in at least one
embodiment of the invention. The appearances of the phrase "in one
embodiment" in various places in the specification are not
necessarily all referring to the same embodiment, nor are separate
or alternative embodiments necessarily mutually exclusive of other
embodiments. The same applies to the term "implementation."
Also for purposes of this description, the terms "couple,"
"coupling," "coupled," "connect," "connecting," or "connected"
refer to any manner known in the art or later developed in which
energy is allowed to be transferred between two or more elements,
and the interposition of one or more additional elements is
contemplated, although not required. Conversely, the terms
"directly coupled," "directly connected," etc., imply the absence
of such additional elements.
The present inventions may be embodied in other specific apparatus
and/or methods. The described embodiments are to be considered in
all respects as only illustrative and not restrictive. In
particular, the scope of the invention is indicated by the appended
claims rather than by the description and figures herein. All
changes that come within the meaning and range of equivalency of
the claims are to be embraced within their scope.
The description and drawings merely illustrate the principles of
the invention. It will thus be appreciated that those of ordinary
skill in the art will be able to devise various arrangements that,
although not explicitly described or shown herein, embody the
principles of the invention and are included within its spirit and
scope. Furthermore, all examples recited herein are principally
intended expressly to be only for pedagogical purposes to aid the
reader in understanding the principles of the invention and the
concepts contributed by the inventor(s) to furthering the art, and
are to be construed as being without limitation to such
specifically recited examples and conditions. Moreover, all
statements herein reciting principles, aspects, and embodiments of
the invention, as well as specific examples thereof, are intended
to encompass equivalents thereof.
It should be appreciated by those of ordinary skill in the art that
any block diagrams herein represent conceptual views of
illustrative circuitry embodying the principles of the invention.
Similarly, it will be appreciated that any flow charts, flow
diagrams, state transition diagrams, pseudo code, and the like
represent various processes which may be substantially represented
in computer readable medium and so executed by a computer or
processor, whether or not such computer or processor is explicitly
shown.
* * * * *
References